Shakespeare wrote of England that it is “set in the silver sea, which serves it in the office of a wall, or as a moat defensive to a house, against the envy of less happier lands.” One result of that geographical happenstance is that the U.K. hasn’t faced the kind of immigration debate that roils the U.S., with its two long, porous land borders, its massive population of illegal immigrants, its tradition of mass immigration and a hitherto remarkable ability to turn newcomers into proud and productive Americans.

But the U.K. is increasingly embroiled in its own, very different debate about immigration, a debate that has come to a head this year thanks to the electoral success of the U.K. Independence Party, which has called for stricter immigration laws and tighter enforcement.

Though Britain experienced waves of invasion and conquest in ancient times, it has never been a “nation of immigrants.” Indeed, most people living in Britain in 1945 could have traced their ancestry back to the Norman Conquest. Between that 1066 invasion and the mid-20th century, there were only two significant arrivals, not counting migration from Ireland. They were the Huguenots in the late 17th century and East European Jews in the early 20th century, and their numbers were actually quite small relative to the overall population of Britain.

In the immediate aftermath of World War II, about 200,000 Poles and other displaced East Europeans settled in the U.K. Then in the late 1940s small numbers of the British Empire’s 800 million diverse subjects began to arrive, encouraged by governments worried about labor shortages. But it was only from the 1950s on that they started arriving in significant numbers. Annual immigration from the Commonwealth reached a peak of 136,000 in 1961. Racial tension, segregation, exploitation, inequality and a host of other ills resulted, climaxing in urban riots in the early and mid-1980s.

Entry requirements for Commonwealth citizens were tightened in the late ’60s and early ’70s, though a controversial exception was made in 1973 for 30,000 Ugandan Indians persecuted and expelled by Idi Amin. They became the most successful immigrant group in British history. Many of the late-night corner shops that so improved life in Britain were theirs, and today they are to be found at the top of many professions.

These days the mostly West Indian immigrants who pioneered immigration into the U.K., and their descendants, are thoroughly integrated into every aspect of British life. So are most of the immigrants who came from India and Pakistan in increasingly large numbers in the 1960s. Self-segregation, misogynistic practices like forced marriage, and Islamist radicalism in some of these communities have at times prompted controversy, as did the Salman Rushdie affair, but in general immigration was a diminishing issue in the U.K. until the late 1990s.

Everything changed with the 1997 election of a Labor government that encouraged a vast and unprecedented wave of mass immigration. Between that year and 2010, about 5.2 million immigrants are known to have come and stayed. The number may be even higher—lax enforcement of immigration laws and the abandonment of departure controls in 1996 make it impossible to know for certain.

Unlike previous large-scale immigration, the post-1997 newcomers often didn’t come from former colonies. Many were from countries with little or no historical or cultural connection to Britain—places like Ethiopia, Morocco, Brazil and Vietnam. The five million that arrived in 15 years brought the British population to 62 million and helped make England the most densely inhabited country in Europe.

There was no public mandate for the new open-door policy, nor for the failure to keep tabs on people given student and other temporary visas, nor for the flaws in the welfare system that inspired thousands of bogus asylum-seekers to gather in northern France to smuggle themselves into the U.K. Indeed, surveys repeatedly showed public unhappiness with all of this. But more or less uncontrolled mass immigration was supported by all the major parties.

The wave of immigrants that swept into the U.K. after 1997 undoubtedly brought many benefits and improved various aspects of British life. Eating habits have changed for the better. Street life is more vibrant. And thanks especially to immigrants from the EU, cafe life is everywhere, and service in the hospitality industry is vastly improved. Cheap child care and catering by minimum-wage nannies and cooks is a boon for the professional classes, especially professional women.

But beyond these goods, the all-round economic fallout is more debatable. There is little or no evidence of an increase in gross domestic product per capita, but there have been demonstrable strains on the education and welfare systems.

More important, modern Britain was and is culturally ill-prepared for such large-scale immigration. Early 20th-century America came up with pledges of allegiance, citizenship ceremonies and a whole battery of other devices—some of them condescending or even racist—to turn the apparently unpromising or even frightening immigrants from Southern and Eastern Europe into Americans.

In 1990s Britain, however, there was no cultural infrastructure designed to enable the absorption and assimilation of this human wave. Nor has there ever been a break in mass immigration, as there was in the U.S. in the 1920s and ’30s, during which those who had arrived in America’s great ports began to take their place in and enrich the wider society.

Despite the official guff about “cool Britannia,” Britain in the 1990s wasn’t a confident society like turn-of-the-20th-century America. It was so uncertain about its values that there was no consensus on whether immigrants should be required to assimilate at all. Much of the rhetoric that celebrated mass immigration objected to the very idea: The vibrant, exciting newcomers had little to learn from (but much to teach!) a bland, uptight, postimperial society responsible for so many of the world’s ills.

It wasn’t surprising that some new immigrants felt little respect for, or obligation to, British society and culture—or that some of their children were drawn to terrorist violence. If British policy makers had paid closer attention to America’s historical experience of immigration and attitudes toward it, this might have been avoided.

—Mr. Foreman is a senior research fellow at the Civitas Institute for the Study for Civil Society in London and co-founder of The Indian Quarterly, a Mumbai-based magazine. (All views expressed here are his own.)